The electrical characteristics and the plasma parameters of planar inductively coupled plasmas (ICP) have been measured. The resistance of the total load including the coil and the plasma varied from 1 to 4 W and the inductance from 1.5 m to 2 mH when the power was changed from 100 to 1000 W and the pressure from 1 to 10 mTorr. The density of electron measured by Langmuir probe was over $10^{11}/ extrm{cm}^3$ and the temperature varied between 3 and 5 eV as the process conditions were changed. Bias modulation was adopted as a new method to improve the selectivity of $SiO_2$on Si in $C_4F_8$ (octafluorocyclobutane) plasma. The selectivity was improved as the duty ratio decreased, but the etch rate of $SiO_2$decreased below 400$AA$/min. $H_2$addition to $C_4F_8$ plasma showed that the etch selectivity could be higher than 50 and the etch rate of $SiO_2$over 2000$AA$/min when 60% $H_2$was added.